1. Field of the Invention
This invention relates to circuit boards and, more particularly, to signal paths which provide improved signal integrity.
2. Description of the Related Art
Circuit boards come in many different types. One common type of circuit board is a printed circuit board. Printed circuit boards generally have one or more layers of insulating or dielectric material which may be laminated together. Each layer may include multiple signal paths or “signal traces” which are used to propagate signals. In addition, some layers may be used only to provide power or ground and may be considered as a solid plane. These types of layers are typically referred to generally as a reference plane or as power and ground planes, respectively. Further, other layers may include both power or ground planes as well as signal traces.
The circuit board is typically made from an insulating material such as fiberglass or similar dielectric material which may provide isolation between signal traces as well as a rigid backing for mechanical strength.
The signal traces are typically thin metallic “wires” which have been etched from a pattern which has been printed onto a metal layer which may be bonded to the surface of the circuit board. The metal is generally copper or some other similar conductive copper alloy. Depending on the type of process used to manufacture the circuit board, the unused metal may be etched away leaving the signal traces and any other metallic contact surfaces intact.
At high frequencies, the traces on a circuit board may act like transmission lines, thus certain trace characteristics become important when considering signal integrity. Generally, for maximum power transfer and minimum signal reflection it is important to match the impedance of the load to the impedance of the source and additionally to the impedance of the transmission line. Depending on how well these impedances are matched may determine how much the input signal is distorted or reflected as it propagates down the trace.
Generally, the traces are routed to have as much space between traces as possible. However, depending on such factors as the density of the signal traces and circuit components, signal traces may necessarily be routed very close together. Depending on the frequency of the routed signals, routing the traces close together may cause problems such as cross-talk, for example. Cross-talk refers to a condition where the coupling capacitance between two adjacent traces allows signals which reach a critical frequency to interfere with each other.
The impedance of the transmission line is generally referred to as the characteristic impedance. The equations that are used to determine the characteristic impedance include incremental values of inductance, capacitance, resistance and leakage which may also be referred to as conductance. The characteristic impedance may be calculated for a given transmission line. Generally, if the transmission line impedance does not match the impedance of the load and the impedance of the source, the behavior of the transmission line may be changed (i.e., tuning) by adding predetermined impedance components along the length of the transmission line.
However, it may be difficult if not impossible to adjust the behavior of a signal trace on a conventional circuit board, particularly a densely populated circuit board. Thus to convey signals at very high frequencies on a conventional circuit board, even a well-routed trace may induce signal distortion due to an impedance mismatch. Therefore, the frequency of operation of a conventional circuit board may be limited.